Measles has been all but eradicated in the developed world, but it still claims more than 160,000 lives in developing countries. Sub-Saharan Africa, in particular, has been hit hard in the past few years. A 2009 outbreak in Zimbabwe, for instance, afflicted 8,000 people and killed 517. Some public health workers blame lax vaccination efforts, but the real culprit may be HIV.

Studies show that infants with HIV do not respond well to the measles vaccine even when given a second dose at nine months, as the World Health Organization (WHO) currently recommends. A 2008 study in Malawi found that the extra dose only boosted measles immunity to 64 percent. Worse yet, a 2009 study in Kenya found that only 33 percent of HIV-infected children who had been vaccinated for measles at birth still had antibodies to the virus at age five. “Normally measles immunity will last 10 years and often for life,” says William Moss, a public health researcher at Johns Hopkins University who studies HIV and measles in Zambia.

The implications for immunization efforts are serious. The current vaccination schedule is inadequate for countries with high HIV levels, says Anna Nilsson, an immunologist at the Karolinska Institute in Sweden and co-author of a March paper on measles and HIV in PLoS Pathogens. “The traditional approach has been to give the same vaccine at the same age to all children,” she says. “But here we have a group of very vulnerable children who need special consideration.” More than two million children in sub-Saharan Africa carry HIV.

Fortunately, HIV-infected children can develop immunity to measles if they receive antiretroviral therapy before vaccination. When the HIV-infected five-year-olds in the Kenya study were revaccinated after six months of antiretroviral treatment, their measles immunity rose to 78 percent. And ­Nilsson and her colleague found that treating HIV-infected infants with anti­retro­virals preserves a type of immune cell that improves vaccine response. “For immunization efforts to be successful in HIV-infected children, you also have to provide anti­retro­virals,” Nilsson says. The WHO is working to make retrovirals more available.* Now they have an added incentive.

*Erratum (April 25, 2011): This sentence should read: "The WHO is working to make antiretrovirals more available.